MPEG defines a set of standards for video compression algorithms commonly used by the broadcast and studios for recording and broadcasting digital encoded video. The broadcast industry immediately recognized the potential of MPEG technology to increase channel efficiency of satellite transponders and cable networks. The MPEG video and audio specifications give the syntax and semantics of encoded video and audio bitstreams necessary for communicating compressed digital audiovideo data, as well as storing such data on media in a standard format. To date, two standards have been announced, ISO/ANSI MPEG1 and MPEG2.
Referring to FIG. 1, a conventional prior art MPEG digital video receiver/decoder 30 receives an encoded digital video signal 34, such as an ISO/ANSI MPEG1 or MPEG2 encoded signal, from a transmission channel 32. A channel specific decoder 36 removes the transmission encoding and delivers an MPFG-encoded signal 38 to an MPEG specific decoder 40. The MPEG specific decoder 40 separates the MPEG-encoded signal into an encoded video signal 42 and an encoded audio signal 44. A video decoder 46 and an audio decoder 48 decode the encoded video and audio signals 42 and 44 to produce digital video and audio signals 50 and 52, respectively.
Digital video encoders must be highly accurate in limiting frequency drift of system clocking signals so that decoders receiving digital video data from the encoders may use such clocking signals to recover encoded video data accurately. For digital video signals in the conventional MPEG formats, the system clock in a typical video encoder must meet the following bounds on its frequency:
Nominal Frequency=90 KHZ PA1 Frequency Drift.ltoreq..+-.14.5 Hz PA1 Rate of Change of Frequency Drift&lt;0.00025 Hz/s PA1 A digital video tape drive may ensure low frequency drift using a mechanism that is independent of encoded program material and video display rate. PA1 The digital video tape drive may be used in applications in which the video display rate is other than the conventional analog NTSC and PAL rates. In particular, the tape drive may be used in applications in which various MPEG frequencies are used. PA1 The digital video tape drive may support different aspect ratios and picture resolutions. PA1 Low frequency drift may be achieved during both recording and playback of digital video data in applications, such as MPEG applications, having various display frequencies.
For best performance, both the bounds on absolute frequency drift and rate of change of frequency drift must be maintained for accurate recording and playback of digital video.
In theory, a digital video tape drive onto which decoded video data is to be recorded should maintain a similar degree of accuracy to ensure that video decoder buffers do not overflow or underflow as video data is decoded during playback. (Simply increasing the size of the decoding buffers to prevent overflow will increase the cost of the system). The tape drive also should maintain very accurate timing during the recording process to ensure proper video playback.
Referring to FIG. 2, a typical video recording/playback system 10 (either analog or digital) includes a video receiver 12 that receives a video signal and processes the signal (e.g., by demodulating it) into a form suitable for recording. The video receiver 12 provides the processed video signal as input to a video recording/playback device, such as a helical scan recorder 14, which records the processed video signal onto a recording medium, such as a video tape 16. A helical scan recording drum 18 driven by a drum motor 19 records the processed video signal onto the video tape 16 as the tape 16 is pulled past the drum 18 by a capstan 20 driven by a capstan motor 21. During playback, the drum 18 retrieves the recorded video signal from the video tape 16 and provides the video signal to a playback circuit 24, which in turn provides an analog video signal to a video display device 26, such as a television monitor. The drum motor 19 controls the rotational speed of the drum 18 and, therefore, the data transfer rate of the recorder 14. The capstan motor 21 controls the rotational speed of the capstan 20 and, therefore, the pitch of the video tracks recorded on the tape 16.
Conventional analog and digital video tape drives maintain accuracy in frequency drift by synchronizing the drum motor 19 and the capstan motor 21 to the eventual analog display rate of the video frames recorded on the tape 16. For example, conventional analog VHS and Beta video recorders use a frame-to-frame recording process in which each picture frame is recorded on an individual analog track at a rate of 30 Hz for video encoded according to the NTSC standard and 25 Hz for video encoded according to the PAI. standard. For NTSC-encoded video, each frame consists of two fields that are displayed at a rate of 60 Hz. Analog video tape drives minimize frequency drift by using a tracking control circuit that locks the video track frequency to the recording/playback line frequency, which is 60 Hz for NTSC and 50 Hz for PAL. However, this technique will not allow digital data to be recorded at different video display rates.
Digital video tape drives also typically minimize frequency drift by operating at a fixed frame transfer rate and locking the video track recording rate to the line frequency. However, this method requires a fixed record/playback rate and does not support different display rates.
The inventors have recognized a need for controlling frequency drift of a digital video tape drive, particularly for such a drive having a variable display rate and interframe compressed video where there is no relationship between track rate and frame display rate. The present invention addresses this need.